There is an increasing requirement to recycle materials such as aluminium, magnesium and other metals and non-metals. Often such materials will be coated in paint, oil, water, lacquers, plastics, or other volatile organic compounds (VOCs) which must be removed prior to re-melting the materials. For materials which are capable of being processed at relatively high temperatures without melting, such impurities are typically removed using a thermal process which is sometimes known as de-coating. Such thermal de-coating processes can also be used to dry and/or sterilise materials prior to remelting.
For example, aluminium is often used in the production of beverage cans which are typically coated in paint, lacquers and/or other VOCs, and also in larger products such as internal combustion scrap. Before the aluminium can be melted down for recycling, any coatings or other impurities must be removed in order to minimise metal loss and the aluminium separated from other metals.
Known de-coating processes involve exposing the material to be treated to hot gases in order to oxidise the coatings and/or impurities which are to be removed. This exposure takes place in a closed environment in which the temperature and oxygen content of the hot gases can be controlled. Temperatures in excess of 300 C are required to remove most organic compounds and an oxygen level in the range of 6% to 12% is normally required.
If the temperature and oxygen levels of the hot gases are not carefully controlled the process can go autothermic as the VOCs which are released during the thermal stripping are combusted. This can result in an uncontrolled increase in the temperature of the hot gases which may be very dangerous.
The material will usually be shredded before treatment and it is important for effective de-coating that all the surfaces of the shredded material are exposed to the hot gases, If this does not occur then the treatment becomes less effective and, in the case of U.B.C.s in particular, a black stain may be left on the surface of the treated material. It is also desirable for the material to be agitated during the treatment to physically remove lose coatings or impurities from the material.
At present the main systems which are used for thermal de-coating and metal recovery are:
1 Static Oven
In a static oven, the material is stacked on a wire mesh and hot gases are recirculated through the oven to heat the material to the required process temperature.
This arrangement is not efficient because the hot gases do not come in to contact with the materials that are enclosed within the stack of materials on the mesh. As discussed previously, it is important in de-coating that all the surfaces of the materials being treated are exposed to the hot gases. Also there is no agitation of the material being treated.
2 Conveying Oven
This system uses a mesh belt conveyor to transport materials for treatment through an oven. Hot gasses are passed through the material on the belt as it passes through the oven. The problems with this method are as follows:                The depth of materials on the belt limits the process. The materials are stacked, causing similar problems to those found with the static oven in which materials at the centre of the stack do not come into contact with the hot gases        There is no agitation of the materials, so loose coatings are not removed.        The conveyor belt life is short.        The materials have to be constantly fed.        The process is not suitable for low volume or continuously changing product.        
3 Rotating Kiln
A large kiln is inclined to the horizontal so that material fed or charged into the kiln at its highest end travels towards the lowest end, where it is discharged, under the influence of gravity. The kiln is rotated so that material within the kiln is agitated and a flow of hot gases is provided to heat up the material as it travels through the kiln. A number of problems are associated with this method:                The material has to be constantly fed.        The process is not suitable for low volume or continuously changing product.        The continuous process requires air locks at both ends, materials charge end and materials discharge end.        The kiln requires a rotating seal leading to a high level of maintenance.        
4 Sloping Hearth Furnace
This is used for reclaiming metal such as aluminium from scrap and the like. A dross furnace is provided with one or more tap holes or a full width slot so sized as to permit the passage of molten metal into a reservoir while retaining the resulting waste material. The dross furnace has a sloping hearth on which the scrap are loaded and the resulting molten aluminium drains into the reservoir. The angle of the sloping hearth is critical and dust and sand from the scrap can contaminate the metal.
WO 01/98092 A1 describes a pivotable or tiltable oven that overcomes many of the disadvantages of the previously known apparatus and methods for thermal de-coating. For a detailed description of the construction and operation of the oven, the reader should refer to WO 01/98092 A1. However, briefly, the oven has a charging portion for receiving material to be treated and a changeover portion. Incorporated within the changeover portion is a heat treatment chamber through which a stream or flow of hot gasses can be passed. The oven is pivotally moveable between a first position in which the changeover portion is higher than the charging portion and a second position in which the charging portion is higher than the changeover portion. The arrangement is such that the oven can be repeatedly moved between the first and second positions so that material within the oven falls from one portion to the other portion, passing through the stream of hot gasses in the heat treatment chamber. A method of using the apparatus is also disclosed.
The above known oven has the advantage that it can be used to treat comparatively low volumes of material in a batch process. A further advantage is that by controlling the movement of the oven, the material being treated can be brought into and out of the heat treatment chamber at will, enabling the oven to be operated safely without the process going autothermic in an uncontrolled manner and allowing a very fine degree of control of the treatment process.
The oven described in WO 01/98092 A1 has been found to work well, providing a commercially and technically acceptable means of thermally de-coating relatively low volumes of materials. However, when treating light weight materials, such as powders or materials that have been shredded into very small pieces, there can be a tendency for some of the material being treated to become entrained in the flow of hot gasses passing through the heat treatment chamber. Whilst some of the entrained material can be filtered out of the gas flow and recollected, there is an overall reduction in the efficiency of the process.
It is an object of the present invention is to provide an improved apparatus for reclaiming aluminium from scrap or waste material and the like.